The present invention relates to hydraulic lifts for watercraft. The prior art includes a number of hydraulic boat lifts. U.S. Pat. No. 5,184,914 to Basta discloses a lift with pivoting booms supported on a rectangular base submerged in water. A boat support frame is attached to the pivoting booms. A double-acting hydraulic jack, a cylinder with piston, swings the booms upwardly to raise a boat out of the water. When the piston is retracted into the cylinder, the booms swing downwardly to lower the boat into the water. This invention does not rely on locking the piston in an extended position so that the lifting booms stay upright even if pressure is lost in the hydraulic jack, but rather uses canted braces engaging one of the lifting booms in an upright position so as to hold a boat up out of the water. U.S. Pat. No. 5,919,000 to Unkle also employs an hydraulic cylinder and piston combination to swing pivoting lifting elements up or down. Unkle relies on a pawl member which engages recesses in the piston shaft to lock it in an extended position in the event pressure in the cylinder is lost. A cord attached to the pawl is used to remove it from a recess so that the piston can retract into the cylinder when it is drained of water. Unkle also has adjustable vertical legs and feet. U.S. Pat. No. 4,895,479 to Michaelsen is yet another lift using an hydraulic cylinder and piston combination, pivoting lifting elements, and a locking mechanism using a compression spring to bias a pawl against the piston shaft.
None of these inventions addresses the issue of deliberate and unwanted lowering of a boat lift in order to steal the boat supported thereon. Accordingly there is a need for a locking device that can be released only by a legitimate user.